1. Field of the Invention
The present invention relates generally to superhard material cutting elements for earth boring drill bits, and specifically to modifications to the geometry of the peripheral cutting edge of such cutting elements.
2. State of the Art
Superhard cutting elements in the form of Polycrystalline Diamond Compact (PDC) structures have been commercially available for approximately two decades, and planar PDC cutting elements for a period in excess of 15 years. The latter type of PDC cutting elements commonly comprises a thin, substantially circular disc (although other configurations are available) including a layer of superhard material formed of diamond crystals mutually bonded under ultrahigh temperatures and pressures and defining a planar front cutting face, a planar rear face and a peripheral or circumferential edge, at least a portion of which is employed as a cutting edge to cut the subterranean formation being drilled by a drill bit on which the PDC cutting element is mounted. PDC cutting elements are generally bonded during formation to a backing layer or substrate formed of tungsten carbide, although self-supporting planar PDC cutting elements are also known, particularly those stable at higher temperatures, which are known as TSP' s, or Thermally Stable Products.
Either type of PDC cutting element is generally fixedly mounted to a rotary drill bit, generally referred to as a drag bit, which cuts the formation substantially in a shearing action through rotation of the bit and application of drill string weight thereto. A plurality of either, or even both, types of PDC cutting elements is mounted on a given bit, and cutting elements of various sizes may be employed on the same bit.
Drag bits may be cast and/or machined from metal, typically steel, or may be formed of a powder metal infiltrated with a liquid binder at high temperatures to form a matrix. PDC cutting elements may be brazed to a matrix-type bit after furnacing, or TSP's may even be bonded into the bit body during the furnacing process. Cutting elements are typically secured to cast or machined (steel body) bits by preliminary bonding to a carrier element, commonly referred to as a stud, which in turn is inserted into an aperture in the face of the bit and mechanically or metallurgically secured thereto. Studs are also employed with matrix-type bits, as are cutting elements secured via their substrates to cylindrical carrier elements affixed to the matrix.
It has long been recognized that PDC cutting elements, regardless of their method of attachment to drag bits, experience relatively rapid degradation in use due to the extreme temperatures and high loads, particularly impact loading, as the bit drills ahead downhole. One of the major observable manifestations of such degradation is the fracture or spalling of the PDC cutting element cutting edge, wherein large portions of the superhard PDC layer separate from the cutting element. The spalling may spread down the cutting face of the PDC cutting element, and even result in delamination of the superhard layer from the backing layer of substrate or from the bit itself if no substrate is employed. At the least, cutting efficiency is reduced by cutting edge damage, which also reduces the rate of penetration of the drill bit into the formation. Even minimal fracture damage can have a negative effect on cutter life and performance. Once the sharp corner on the leading edge (taken in the direction of cutter movement) of the diamond table is chipped, the amount of damage to the table continually increases, as does the normal force required to achieve a given depth of cut. Therefore, as cutter damage occurs and the rate of penetration of the drill bit decreases, the standard rig-floor response of increasing weight on bit quickly leads to further degradation and ultimately catastrophic failure of the chipped cutting element.
It has been recognized in the machine-tool art that chamfering of a diamond tool tip for ultrasonic drilling or milling reduces splitting and chipping of the tool tip. J. Grandia and J. C. Marinace, "DIAMOND TOOL-TIP FOR ULTRA-SONIC DRILLING"; IBM Technical Disclosure Bulletin Vol. 13, No. 11, April 1971, p. 3285. Use of bevelling or chamfering of diamond and cubic boron nitride compacts to alleviate the tendency toward cutter edge chipping in mining applications was also recognized in U.K. Patent Application GB 2193749 A.
U.S. Pat. No. 4,109,737 to Bovenkerk discloses, in pertinent part, the use of pin- or stud-shaped cutting elements on drag bits, the pins including a layer of polycrystalline diamond on their free ends, the outer surface of the diamond being configured as cylinders, hemispheres or hemisphere approximations formed of frustoconical flats.
U.S. Pat. No. Re. 32,036 to Dennis discloses the use of a bevelled cutting edge on a disc-shaped, stud-mounted PDC cutting element used on a rotary drag bit.
U.S. Pat. No. 4,987,800 to Gasan, et al. references the aforementioned Dennis reissue patent, and offers several alternative edge treatments of PDC cutting elements, including grooves, slots and pluralities of adjacent apertures, all of which purportedly inhibit spalling of the superhard PDC layer beyond the boundary defined by the groove, slot or row of apertures adjacent the cutting edge.
Finally, U.S. Pat. No. 5,016,718 to Tandberg discloses the use of planar PDC cutting elements employing an axially and radially outer edge having a "visible" radius, such a feature purportedly improving the "mechanical strength" of the element.
In summary, it appears that if the initial chipping of the diamond table cutting edge can be eliminated, the life of a cutter can be significantly increased. Modification of the cutting edge geometry was perceived to be a promising approach to reduce chipping, but has yet to realize its full potential in prior art configurations.